Orgnic polymer solar cells are fabricated with the poly ［N-9′-heptadecany 1-2,7-carbazole-alt-5, 5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)］ (PCDTBT) with lower bandgap than poly (3-hexylthiophene) (P3HT) parforming as electron donor material and (［6,6］-phenyl C70-butyric acid methyl ester) (PC70BM) instead of [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), which has wider spectra absoption capability as electron accepter. The power conversion efficiency (PCE) has enhanced to 5.65% by controlling the growth rate and the environment of active layer. Then the thickness of active layer film is optimized, the PCE is improved to 5.84% with the short-circuit current density of 14.2 mA/cm2. The results demonstrate that even without optical spacer the active layer can also have a higher absoption rate and PCE with the control of growth rate and the thickness of active layer.